My S3a has the same floor, I ran 2 lengths of angle front to rear for both the passenger and drivers side picking up on the L to R forward crossmember and the rear tub lip behind the seats. The seat runners were bolted through the angle iron, this stiffens up the whole tub.
I also found that the rear floor stiffeners under the boot around the diff started to crack so I made a box section that bolted below and above the boot floor which stops the flexing.